Theologically, the church is associated with the doctrine of Nestorianism, leading to the church, also being known as a "Nestorian Church", though church leadership has at times rejected the Nestorian label, and was already extant some four centuries prior to Nestorius. The church employs the Syriac dialect of the Aramaic language in its liturgy, the East Syrian Rite, which includes three anaphoras, attributed to Saints Addai and Mari, Theodore of Mopsuestia and Nestorius.[6]

The Church of the East developed between the 1st and 3rd centuries AD from the early Assyrian Christian communities in the Assuristan province (Parthian ruled Assyria) of the Parthian Empire, and at its height had spread from its north Mesopotamian heartland to as far as China, Central Asia and India. A dispute over patriarchal succession led to the Schism of 1552, resulting in there being two rival Patriarchs. One of the factions that eventually emerged from this split became the Assyrian Church of the East, while another became the church now known as the Chaldean Catholic Church, originally called The Church of Athura (Assyria) and Mosul, which eventually entered into communion with the Catholic Church, both in continuation from the Church of the East.

A more recent schism in the church resulted from the adoption of the Assyrian Church of the East of the Gregorian Calendar rather than maintaining the traditional Julian calendar that is off by 13 days. The opponents to the reforms introduced formed in 1964 the Ancient Church of the East headquartered in Baghdad and headed since 1968 by a separate Catholicos-Patriarch.

The Church of the East originally developed during the 1st century AD in the MesopotamianEastern Aramaic speaking regions of Assyria and northwestern Persia (today's Iraq, southeast Turkey, northeast Syria and north western Iran), to the east of the Roman-Byzantine empire. It is an Apostolic church, established by the apostles St Thomas (Mar Toma), St Thaddeus (Mar Addai), and St Bartholomew (Mar Bar Tulmay). St Peter (Mar Shimun Keapa), the chief of the apostles added his blessing to the Church of the East at the time of his visit to the See at Babylon, in the earliest days of the church when stating, "The elect church which is in Babylon, salutes you; and Mark, my son (1 Peter 5:13).[7]

Official recognition was first granted to the Christian faith in the 4th century with the accession of Yazdegerd I to the throne of the Sassanid Empire. In 410, the Synod of Seleucia-Ctesiphon, held at the Sassanid capital, allowed the Church's leading bishops to elect a formal Catholicos, or leader. The Catholicos, Mar Isaac, was required both to lead the Assyrian Christian community, and to answer on its behalf to the Sassanid Emperor.[8][9]

Under pressure from the Sassanid Emperor, the Church of the East sought increasingly to distance itself from the western (Roman Empire) Catholic Church. In 424, the bishops of the Sassanid Empire met in council under the leadership of Catholicos Mar Dadisho I (421–456) and determined that they would not, henceforth, refer disciplinary or theological problems to any external power, and especially not to any bishop or Church Council in the Roman Empire.[10]

As such, the Mesopotamian and Assyrian Churches were not represented at the various Church Councils attended by representatives of the Western Church. Accordingly, the leaders of the Church of the East did not feel bound by any decisions of what came to be regarded as Roman Imperial Councils. Despite this, the Creed and Canons of the first Council of Nicea (325); affirming the full divinity of Christ; were formally accepted at the Synod of Seleucia-Ctesiphon.[11] The Church's understanding of the term 'hypostasis' differs from the definition of the term offered at the Council of Chalcedon. For this reason, the Assyrian Church has never approved the Chalcedonian definition.[11]

The theological controversy that followed the First Council of Ephesus, in 431, proved a turning point in the Church's history. The Council condemned as heretical the Christology of Nestorius, whose reluctance to accord the Virgin Mary the title 'Theotokos' ('God-bearer' or 'Mother of God') was taken as evidence that he believed two separate persons (as opposed to two united natures) to be present within Christ. (For the theological issues at stake, see Assyrian Church of the East and Nestorianism.)

The Sassanid Emperor, hostile to the Roman Empire, saw the opportunity to ensure the loyalty of his Christian subjects and lent support to the Nestorian schism. The Sassanid Emperor took steps to cement the primacy of the Nestorian party within the Church of the East, granting its members his protection,[12] and executing the pro-Roman Catholicos Babowai, replacing him with the Nestorian Bishop of Nisibis, Barsauma. The Catholicos-Patriarch Mar Babai I (497–503) confirmed the association of the Persian Church with Nestorianism.

During the medieval period the geographical horizons of the Church of the East extended well beyond its heartland in present-day northern Iraq. Communities sprang up throughout Central Asia, and missionaries from Assyria and Mesopotamia took the Christian faith as far as China and the Malabar Coast of India.[13]

The massacres of Assyrian Christians by Tamerlane (1336–1405) destroyed many bishoprics, including the ancient Assyrian city of Ashur. The Church of the East, which had previously extended as far as China, was largely reduced to an Eastern Aramaic speaking Assyrian remnant living in its original heartland in Upper Mesopotamia (what had been Assyria), the triangular area[14] between Amid, Salmas and Mosul. The See was moved to the Assyrian town of Alqosh, in the Mosul region, and Mar Shimun IV Basidi (1437–1493) appointed Patriarch, establishing a new, hereditary, line of succession.[15]

Growing dissent in the church's hierarchy over hereditary succession came to a head in 1552, when a group of bishops from the Northern regions of Amid and Salmas elected Mar Yohannan Sulaqa as a rival Patriarch. Seeking consecration as Patriarch by a Bishop of Metropolitan rank, Sulaqa traveled to Rome in 1553, and entered into communion with the Roman Catholic Church. On being appointed Patriarch, Sulaqa took the name Mar Shimun VIII and was granted the title of "Patriarch of Mosul and Athur (Assyria)". Later this title became "Patriarch of the Chaldeans", despite none of its adherents being from the long disappeared Chaldean tribe, or from what had been South in the far south east of Mesopotamia.[16]

Mar Shimun VIII Yohannan Sulaqa returned to the Near East the same year, establishing his seat in Amid. Before being put to death by partisans of the Patriarch of Alqosh, he ordained five metropolitan bishops, thus establishing a new ecclesiastical hierarchy, a line of patriarchal descent known as the Shimun line.

Relations with Rome weakened under Shimun VIII's successors, all of whom took the name Shimun. The last of this line of Patriarchs to be formally recognized by the Pope died in the early 17th century. Hereditary accession to the office of Patriarch was reintroduced, and by 1660 the Assyrian Church of the East had become divided into two Patriarchates; the Eliya line, based in Alqosh (comprising that portion of the faithful which had never entered into Communion with Rome), and the Shimun line.

In 1672[15] the Patriarch of the Shimun line, Mar Shimun XIII Denha, moved his seat to the Assyrian village of Qochanis in the mountains of Hakkari. In 1692, the Patriarch formally broke communion with Rome and allegedly resumed relations with the line at Alqosh, though retaining the independent structure and jurisdiction of his line of succession.

The so-called Chaldean Patriarchate was revived in 1672 when Mar Joseph I, then the Assyrian Church of the East metropolitan of Amid, entered into communion with Rome, thus separating from the Patriarchal See of Alqosh. In 1681, the Holy See granted Mar Joseph the title of "Patriarch of the Chaldeans deprived of its Patriarch", thus forming the third Patriarchate of the Assyrian Church of the East. It was this third Patriarchate that was to become known as the Chaldean Catholic Church in 1683.

Each of Joseph I's successors took the name Joseph. The life of this Patriarchate was difficult; stricken early on with internal dissent, the Patriarchiate later struggled with financial difficulties due to the tax burden imposed by the Turkish Ottoman Empire. Despite these difficulties, the influence of the Patriarchate expanded from its original homeland of Amid and Mardin towards the area of Mosul, where ultimately the See was relocated.

Mar Yohannan VIII Hormizd, the last of the Eliya hereditary line of the Assyrian Church of the East in Alqosh, made a Catholic profession of faith in 1780. Though entering full communion with the Roman See in 1804, he was not recognized as Patriarch by the Pope until 1830. This move merged the majority of the Patriarcate of Alqosh with the Josephite line of Amid, thus forming the modern Chaldean Catholic Church.

The Shimun line of Patriarchs, based in Qochanis, remained within the Assyrian Church of the East, and refused to enter communion with Rome and join the Chaldean Church. The Patriarchate of the present-day Assyrian Church of the East, with its see in Chicago, forms the continuation of this line.[17]

In spite of both ethnic and religious persecution and a serious decline in membership since their height around the fourth century, the Assyrian Church of the East has survived into the 21st century. Here is St. Mary Assyrian Church in Moscow.

In 1915 the Assyrian Church see at Qochanis see was completely destroyed by the Ottoman Turkish Empire in the context of the Assyrian Genocide, Assyrian war of independence and Armenian Genocide. Survivors of the massacres escaped by marching over the mountains into Iran and Iraq to join their kinsmen. In 1918, after the murder of Mar Shimun XXI Benyamin and 150 of his followers, and fearing further massacres at the hands of the Turks and Kurds, the survivors fled from Iran into what was to become Iraq, seeking protection under the British mandate there, and joining ancient indigenous existing Assyrian communities of both Eastern Rite and Catholic persuasions in the north of that country.[18]

The British administration employed Assyrian troops (Assyrian Levies) to put down Arab and Kurdish rebellions in the aftermath of World War I. In consequence, Assyrians of all denominations endured persecution under the Hashemite monarchy, leading many to flee to the West, in particular to the United States, where Chicago became the center of the diaspora community.

During this period the British-educated Patriarch Mar Eshai Shimun XXIII, born into the line of Patriarchs at Qochanis, agitated for an independent Assyrian state. Following the end of the British mandate in 1933[18] and a massacre of Assyrian civilians at Simele by the Iraqi Army, the Patriarch was forced to take refuge in Cyprus.[19] There, Shimun petitioned the League of Nations regarding his peoples' fate, but to little avail, and he was consequently barred from entering Syria and Iraq. He traveled through Europe before moving to Chicago in 1940 to join the growing Assyrian diaspora community there.[19]

The Church and the Assyrian community in general faced considerable fragmentation and upheaval as a result of the conflicts of the 20th century, and Patriarch Mar Eshai Shimun XXIII was forced to reorganize the church's structure in the United States. He transferred his residence to San Francisco, California in 1954, and was able to travel to Iran, Lebanon, Kuwait, and India, where he worked to strengthen the church.[20]

In 1964 he decreed a number of changes to the church, including liturgical reform, the adoption of the Gregorian calendar, and the shortening of Lent. These changes, combined with Shimun's long absence from Iraq, caused a rift in the community which led to another schism. In 1968 traditionalists within the church elected Mar Thoma Darmo as a rival patriarch to Shimun XXIII Eshai, creating the Ancient Church of the East.[21]

In 1972, Shimun decided to step down as Patriarch, and the following year, he married, in contravention to longstanding church custom. This led to a synod in 1973 in which further reforms were introduced, most significantly including the permanent abolition of hereditary succession a practice introduced in the middle of the fifteenth century by the patriarch Shemʿon IV Basidi who had died in 1497); however, it was decided that Shimun should be reinstated. This matter was to be settled at additional synods in 1975, however Shimun was assassinated by an estranged relative before this could take place.[22]

Patriarch Mar Dinkha IV

In 1976, the current Patriarch of the Assyrian Church of the East, Mar Dinkha IV, was elected as Shimun XXIII Eshai's successor. The 33-year old Dinkha had previously been Metropolitan of Tehran, and operated his see there until the Iran–Iraq War of 1980–1988. Thereafter, Mar Dinkha IV went into exile in the United States, and transferred the patriarchal see to Chicago.[23] Much of his patriarchate has been concerned with tending to the Assyrian diaspora community and with ecumenical efforts to strengthen relations with other churches.[23]

The Nestorian nature of Assyrian Christianity remains a matter of contention. Elements of the Nestorian doctrine were explicitly repudiated by Patriarch Mar Dinkha IV on the occasion of his accession in 1976.[24]

The Christology of the Church of the East has its roots in the Antiochene theological tradition of the early Church. The founders of Assyrian theology are Diodorus of Tarsus and Theodore of Mopsuestia, both of whom taught at Antioch. 'Antiochene' is a modern designation given to the style of theology associated with the early Church at Antioch, as contrasted with the theology of the church of Alexandria.[25]

Antiochene theology emphasised Christ's humanity and the reality of the moral choices he faced. In order to preserve the impassibility of Christ's Divine Nature, the unity of His person was defined in a looser fashion than in the Alexandrian tradition.[25] The normative Christology of the Assyrian church was written by Babai the Great (551–628) during the controversy that followed the First Council of Ephesus (431). Babai held that within Christ there exist two qnome (essences, or hypostases), unmingled, but everlastingly united in the one prosopon(personality).

The precise Christological teachings of Nestorius are shrouded in obscurity. Wary of monophysitism, Nestorius rejected Cyril's theory of a hypostatic union, proposing instead a union of will. Nestorianism has come to mean dyaphysitism, in which Christ's dual natures are eternally separate, though it is doubtful whether Nestorius ever taught such a doctrine. Nestorius' rejection of the term Theotokos ('God-bearer', or 'Mother of God') has traditionally been held as evidence that he asserted the existence of two persons – not merely two natures – in Jesus Christ, but there exists no evidence that Nestorius denied Christ's oneness.[26] In the controversy that followed the Council of Ephesus, the term 'Nestorian' was applied to all upholding a strictly Antiochene Christology. In consequence the Church of the East was labelled 'Nestorian', though its theology is not dyophysite.

Pope John XXIII invited many other Christian denominations, including the Assyrian Church of the East, to send "observers" to the Second Vatican Council (1962–1965). These observers, graciously received and seated as honored guests right in front of the podium on the floor of the council chamber, did not formally take part in the Council's debate, but they mingled freely with the Catholic bishops and theologians who constituted the council, and with the other observers as well, in the break area during the council sessions. There, cordial conversations began a rapproachment that has blossomed into expanding relations among the Catholic Church, the Churches of the Orthodox Communion led by the Ecumenical Patriarch of Constantinople, and the ancient churches of the East.

This joint synodal decree provides that (1) Assyrian faithful may participate and receive Holy Communion in a Chaldean celebration of the Holy Eucharist, (2) Chaldean catholic faithful may participate and receive Holy Communion in an Assyrian Church celebration of the Holy Eucharist, even if celebrated using the Anaphora of Addai and Mari in its original form, and (3) Assyrian clergy are invited (but not obliged) to insert the institution narrative into the Anaphora of Addai and Mari when Chaldean faithful are present. Far from expressing a relationship of full communion between these churches, however, the joint synodal decree actually identifies several issues that require resolution to permit a relationship of full communion.

From a Catholic canonical point of view, provisions of the joint synodal decree are fully consistent with the provisions of canon 671 of the 1991 Code of Canons of the Eastern Churches, which states: "If necessity requires it or genuine spiritual advantage suggests it and provided that the danger of error or indifferentism is avoided, it is permitted for Catholic Christian faithful, for whom it is physically or morally impossible to approach a Catholic minister, to receive the sacraments of penance, the Eucharist and anointing of the sick from non-Catholic ministers, in whose Churches these sacraments are valid. 3. Likewise Catholic ministers licitly administer the Sacraments of Penance, the Eucharist and Anointing of the Sick to Christian faithful of Eastern Churches, who do not have full communion with the Catholic Church, if they ask for them on their own and are properly disposed." Canons 843 and 844 of the Code of Canon Law make similar provisions for the Latin Church. The Assyrian Church of the East follows an Open Communion approach allowing any baptized Christian to receive its Eucharist,[28] so there is also no alteration of Assyrian practice. Nonetheless, from an ecumenical perspective, the joint synodal decree marks a major step toward full mutual collaboration of both churches in the pastoral care of their members.

The Church is governed by an episcopal polity, which is the same as other Catholic churches. The church maintains a system of geographical parishes organized into dioceses and archdioceses. The Catholicos-Patriarch, currently Mar Dinkha IV is head of the church. The Synod comprises Bishops who oversee individual dioceses, and Metropolitans who oversee episcopal dioceses in there territorial jurisdiction.

The Chaldean Syrian Church in India and the Persian Gulf is the largest diocese of the church. Its story goes back to the Church of the East that established presence in Kerala. The converts were from lower, untouchable castes, for in a caste-ridden Malabar society. During times of disturbances in the Persian Empire and the Middle East, Assyrian inflow into Kerala ceased and local converts had to take responsibility for the churches. Nevertheless, Malabar churches retained their Nestorian connections. Connection between the Malabar church and the Church of the East was sporadic for a long period till the arrival of the Portuguese. The church is represented by the Assyrian Church of the East and is in communion with it.

Hierarchy

The current hierarchy and dioceses is as follows. The Patriarchate of the Church of the East was located for centuries in the cathedral church of Mar Shallita, in the village of Qudshanis in the Hakkari mountains, Ottoman Empire. After the exodus in 1915 the Patriarchs temporarily resided between Urmia and Salmas, and from 1918 the patriarchs resided in Mosul, Iraq. After the Simele massacre of 1933, the then Patriarch Shimun XXIII Eshai was exiled to Cyprus. In 1940 he was welcomed to the United States where he set up his residence in Chicago, Illinois and administrated the United States and Canada as his Patriarchal province. The patriarchate was moved to Modesto, California in 1954, and finally to San Francisco, California in 1958 due to health issues. After the assassination of the Patriarch and the election of Mar Dinkha IV in 1976, the patriarchate was temporarily located in Tehran, Iran where the patriarch already resided. Since 1980, the Patriarchate again returned to Chicago, Illinois where it currently remains. The Diocese of Eastern United States served as the patriarch's province from 1994 until 2012.

Due to the unstable political, religious and economic situation in the church's historical homeland of the Middle East, many of the church members now reside in Western countries. Churches and dioceses have been established throughout Europe, America and Oceania. The largest expatriate concentration of church members is in the United States, mainly situated in Illinois and California.

Diocese of Syria – jurisdiction lies throughout all Syria, particularly in the Al-Hasakah governorate, where most of the community reside in Al-Hasakah, Qamishli and the 35 villages along the Khabur river. There are also small communities in Damascus and Aleppo

Diocese of Eastern USA – formerly the Patriarchal Archdiocese from 1994 until 2012 . The territory includes the large Illinois community, along with smaller parishes in Michigan, New England and New York.

Archdiocese of Iraq – covers the indigenous territory of the church in Iraq except Northern areas. The archdiocese's territory includes the cities and surroundings of Baghdad, Basra, Kirkuk, and Mosul.

Archdiocese of Nohadra – covers the indigenous territory of the church in Dohuk, Arbil, and Sulaymaniyah in Kurdish northern Iraq.

Diocese of Eastern USA – formerly the Patriarchal Archdiocese from 1994 until 2012 . The territory includes the large Illinois community, along with smaller parishes in Michigan, New England and New York.

Seleznyov, Nikolai N., "Nestorius of Constantinople: Condemnation, Suppression, Veneration, With special reference to the role of his name in East-Syriac Christianity" in: Journal of Eastern Christian Studies 62:3–4 (2010): 165–190.